The main belief of Sikhism is the concept of "Ik Onkar," which translates to "One God." This belief is rooted in the idea that there is only one supreme being, and all other deities or idols are false. Sikhs believe that this one God is formless, eternal, and all-powerful, and is the creator of the universe.

This belief is reflected in the first line of the Sikh scripture, the Guru Granth Sahib, which states, "There is only one God, His name is Truth, He is the Creator, He fears none, He is without hate, He never dies, He is beyond the cycle of births and death, He is self-illuminated, He is realized by the kindness of the True Guru. He was True in the beginning, He was True when the ages commenced and has ever been True, He is also True now." This verse encapsulates the core belief of Sikhism in the oneness of God.

In addition to the belief in one God, Sikhism also places a strong emphasis on service to humanity. This is known as "seva" and is considered an essential aspect of the Sikh way of life. Sikhs believe that by serving others, they are serving God, and it is their duty to help those in need, regardless of their religion, caste, or social status.

This belief is reflected in the teachings of the Sikh Gurus, who emphasized the importance of selfless service and helping those in need. Guru Nanak, the founder of Sikhism, famously said, "The highest religion is to rise to universal brotherhood; to consider all creatures your equals."

Sikhs also believe in the concept of "Sarbat da Bhala," which means "blessings for everyone." This belief promotes the idea of working for the welfare of all and not just for oneself. It encourages Sikhs to be selfless and compassionate towards others, regardless of their differences.

The belief in one God and service to humanity is also reflected in the daily prayers and rituals of Sikhs. They start their day by reciting the Mool Mantar, which reaffirms their belief in one God and the importance of service to others. Sikhs also engage in community service, such as preparing and serving free meals in the Gurudwara (Sikh place of worship), known as Langar, to anyone who visits, regardless of their background.

In conclusion, the main belief of Sikhism is the devotion to one God and service to humanity. This belief is deeply ingrained in the Sikh way of life and is reflected in their daily practices and teachings. Sikhs strive to live a life of humility, compassion, and selfless service, guided by their belief in the oneness of God and the importance of serving others.